(1) Field of the Invention
The present invention relates to an insulating film for use in motor of refrigeration compressor. More particularly, the present invention relates to an insulating film for use in motor of refrigeration compressor, which has excellent resistances to the substitute flon and refrigerating machine oil (e.g. synthetic lubricating oil) used in particular combination in the refrigeration compressor and moreover imparts deterioration resistance to the refrigerating machine oil.
(2) Description of the Prior Art
Flon gases, which have been used as a refrigerant for air conditioner or refrigerating machine, are believed to be a cause for ozone layer destruction and global warming, and the use of particular flons having a high ozone depletion potential, such as R11 (CCl.sub.3 F), R12 (CCl.sub.2 F.sub.2) and the like must be totally banned by 2,000 according to the decision made at the Copenhagen Agreement in November 1992, amended in the Montreal Protocol. In this connection, change of flon gases to substitute flons such as hydrofluorocarbons (HFC), hydrochlorofluoro-carbons (HCFC) and the like is taking place. Of the substitute flons, R134a (CH.sub.2 F--CF.sub.3) is drawing high attention and is coming to be used practically.
R134a, however, is not compatible with mineral oils which have been used as a lubricating oil for refrigeration compressor. Hence, synthetic oils of polyoxyalkylene glycol type, ester-modified polyoxyalkylene glycol type, ester type or the like are coming to be used as a new lubricating oil for refrigeration compressor.
The motor of air conditioner or refrigerating machine is used as a motor of compressor for refrigerant and is constantly placed in an atmosphere where the above-mentioned refrigerant and lubricating oil coexist. Therefore, when the compressor is in operation, the coil of the motor comes in contact with a gaseous refrigerant of high temperature and high pressure, a lubricating oil containing a large amount of the refrigerant dissolved therein, etc.; when the compressor is in stop, a liquid-state refrigerant is accumulated inside the compressor. Thus, the insulating film used for insulation of the above motor is exposed to such conditions over a long period of time and yet is required to have a semi-permanent life.
As such an insulating film, there has been widely used a polyethylene terephthalate (hereinafter abbreviated to PET) film for its balanced properties in heat resistance, solvent resistance, electrical insulation, mechanical strengths, etc. The PET film, however, is liable to hydrolysis by the water present in refrigerating machine oil of polyalkylene glycol type, ester-modified polyalkylene glycol type, ester type or the like, and has inevitably shown significant reduction in mechanical strengths and electrical insulation.
Also in the PET film, the low-molecular materials, etc. contained therein are extracted by the above-mentioned synthetic lubricating oil having a higher solvency for organic substances than mineral oils have, resulting in sludge formation and film embrittlement; thus, there has been a fear of reduction in compressor life.
Recently, as the output of compressor has become larger, the heat resistance and pressure resistance requirements for the insulating film of compressor motor have also become larger. In this connection, study on use of polyimide film, polyamideimide film or the like is under way. These films, however, are not satisfactory because they cause hydrolysis by the water contained in the refrigerating machine oil used in combination with a substitute flon and thereby generate organic acids, which may invite deterioration of the refrigerating machine oil and resultant reduction in refrigerating capacity.